Instead of crashing when an empty key is passed to
EVP_marshal_public_key(), return with an
EVP_R_UNSUPPORTED_ALGORITHM_ERROR. This brings e.g. X509_PUBKEY_set()
closer to how it behaved before 68772b31 (previously, it returned an
error on an empty public key rather than dereferencing pkey->ameth).
Change-Id: Ieac368725adb7f22329c035d9d0685b44b885888
Reviewed-on: https://boringssl-review.googlesource.com/7351
Reviewed-by: David Benjamin <davidben@google.com>